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Warts are circumscribed cutaneous excrescences or lesions of the skin or mucosa having a papilliferous surface and may be rounded, flat, soft, hard, acuminate or filiform. They are an extremely common occurrence, with most people experiencing them in one form or another at some stage in their lives. They are caused by infection of epithelial cells (cells that form the outer layers of the skin or the lining of body cavities) with a papilloma virus, and in humans, with the Human Papilloma Virus (HPV). Viral replication occurs in fully differentiated epithelium and the ensuing proliferation results in a clinically evident papule or plaque. Any epithelial surface can be affected. Infection is through contact with an infected individual or with an object touched by an infected individual, bathrooms and swimming pools being frequent infective zones. An infected individual may spread warts to uninfected areas of the body through scratching or rubbing existing warts and then touching other areas of skin or mucosa (autoinnoculation). The virus enters the skin or mucous membrane through cuts, abrasions, or other surface disruptions. In the skin the virus replicates primarily in the upper layers of the epidermis in differentiated cells. Following a latency period of several months, and less commonly as much as several years, tiny protuberances appear in the affected area. In the skin, these are hyperkeratotic lesions. The protuberances slowly grow and, in the most common form, result in grayish, rough, rounded structures, though other varieties distinguished by shape and color also occur.
Warts can appear at various sites on the body, most commonly on the skin, genitals and mouth, with their clinical appearance depending to some extent on the type of virus involved and the anatomical site. In humans, common warts (verruca vulgaris) are hyperkeratotic, exophytic and dome-shaped papules or nodules, most commonly located on fingers, hands, knees, elbows or other sites of trauma. Their surface is generally rough and commonly fissured or scaly. Less frequently, they may have a more complex branched or cauliflower-like structure. Two or more papules that are close together sometimes coalesce. Plane or flat warts (also known as juvenile warts; verruca plana juvenilis) are flat topped papules with minimal scaling and only slight elevation (generally about 2-4 mm in diameter). Flat warts are smoother and less elevated than common warts. They tend to be multiple and abundant, sometimes forming large groups of coalescing lesions. Myrmecia are large, deep burrowing warts. They tend to cause more inflammation and pain than other varieties of warts. On the surface they are generally round and dome shaped. They occur mostly on the soles of the feet, the palms, around or under the nails, or less commonly on the face or elsewhere. Myrmecia warts are histologically characterized by an abundance of eosinophilic inclusions. Plantar warts (verruca plantaris) can be painful due to their callused, endophytic papules that have deeply penetrating sloping sides and a central depression. Numerous coalesced warts on the plantar surface will form a tile-like pattern known as mosaic warts. Filiform warts are most often seen on the face with characteristic frond-like projections that exhibit quick proliferation. Periungual warts occur anywhere along the nail margins and can subsequently lead to onychodystrophy from nail matrix damage and onycholysis from nail bed warts. Butcher's warts generally resemble common warts, but with a greater tendency to form complex branched and cauliflower-like structures. They are particularly common around the fingernails. Anogenital warts (venereal or condyloma acuminate) occur in the perineum and on the genitalia or in the genital tract. Warts of the genital tract present a serious health risk to women, being associated with squamous cell- and adeno-carcinomas of the cervix. Oral warts are small pink or white papules in the oral mucosa (M. M. Lipke, Clinical Medicine & Research, 2006, 4, 273-293).
Over 80 different types of HPV have been characterized, with several others reported. The most common infections on the hands and feet (verruca vulgaris) are commonly caused by types 1, 2, 3, 4, 7, 26, 27, 29 and 57. Plane or flat warts are distinct from common warts and are generally caused by types 3, 10, 28 or possibly 41. Plantar warts are generally associated with types 1, 2, and 4. Myrmecia warts are caused mainly by type 1, and less commonly by types 2, 3, 4, 27, 29, and 57. Types 6 and 11 are associated with low risk anogenital warts, and types 16, 18, 31, 33, 35, 45, 58, 59 and 66 are commonly associated with squamous cell- and adeno-carcinomas of the cervix.
Warts caused by papilloma virus infection are not limited to humans. Bovine papilloma virus (BPV) is a group of viruses that are highly prevalent in cattle, causing warts of the skin and alimentary tract, as well as teats and udders. Warts caused by BPV-1, 2 and 5 generally have a nodular appearance, with warts caused by BPV-3, 4 and 6 generally having a cauliflower-like appearance, occurring most commonly on the head, neck and shoulders. Although generally harmless, they are unsightly in show animals and large warts may bleed, potentially leading to secondary infections. Florid warts of the teat can cause mastitis and interfere with suckling and milking. Transmission between animals is common, via, for example fences or halters and warts on the teats of lactating cows are readily transmitted to calves via abrasions. Similarly, equine papilloma virus (EPV) cause cauliflower-like growths usually found around the muzzle, lips, nostrils, eyes and occasionally, lower legs, and cross infection occurs readily via fences, halters and feed buckets. Infection by the canine oral papilloma virus (COPV) causes cauliflower-like growths around the lips and muzzle of young animals and on rare occasions, have led to malignancy. Secondary bacterial infections can also occur as a result. Warts arising from papilloma virus infections have been observed for many other animals.
Currently, there is no cure for papilloma virus infection, with therapy aimed at reducing or eliminating symptoms. Non-genital warts in otherwise healthy people or animals are generally harmless and usually resolve spontaneously (within months or years) due to natural immunity. That said, some viral warts persist for many years and may represent a source of further infection. In addition, whilst they are generally harmless, non-genital warts, depending on their location, can be considered cosmetically undesirable, (especially when located on the face or hands) or painful (such as on the soles of the feet and near the nails). Genital warts, however, are sexually transmissible and can, if left untreated, cause cervical cancer in women. In animals, they may interfere with milking, breeding, eating, harnessing and other activities.
Current treatments for warts vary depending on any number of factors, including the size, number and nature of the warts involved, anatomical location, and the health and age of the patient. Treatments include: occlusive duct tape therapy, surgical cutterage or cautery, cryotherapy, chemical therapy, such as topical application of cantharidin, salicylic acid, glutaraldehyde, formaldehyde, formic acid, or silver nitrate, and immunotherapy ((M. M. Lipke, Clinical Medicine & Research, 2006, 4, 273-293 and Gibbs, S, and Harvey, I., Topical Treatments for cutaneous warts (Review), The Cochrane Library, 2006, Issue 3). However, no one therapy is fully effective in all patients.